1. Field of the Invention
The present invention relates to a disk drive having a head that reads data written on a recording surface of a disk. In particular, the present invention relates to an apparatus and method for controlling a head unload operation in a disk drive, which apparatus and method are suitable for reducing power consumption when the head is unloaded in a particular area away from the recording surface of the disk.
2. Description of the Related Art
Recent disk drives, for example, hard disk drives generally comprise ramps. The ramp provides an area to which a head is retracted. The ramp is located away from a recording surface of a disk. The head is supported by a suspension. The suspension is formed at a tip of an actuator. The actuator moves the head in a radial direction of the disk. The actuator includes a voice coil motor (VCM) that drives the actuator. The voice coil motor includes a magnet (permanent magnet) that produces magnetic fields. A tab is formed at a tip of the suspension. Actually, the tab is retracted to the ramp. However, for simplification, a common description is that the head is retracted to the ramp. An operation of retracting the head to the ramp is called an “unload” (head unload). In contrast, an operation of moving the head from the ramp onto the recording surface is called a “load” (head load).
A torque TU applied to the actuator during a head unload operation is expressed by:TU=FUL1  (1)where FU denotes a force resulting from the friction between the ramp and the tab during the head unload operation. L1 denotes the distance between the rotation center of the actuator and the tab (that is, the length of an arm of the actuator).
To enable a head unload operation, it is necessary that a torque TVCM generated by the voice coil motor is higher than the torque TU, applied to the actuator during the head unload operation. The torque TVCM is expressed by:TVCM=FVCML2  (2)where FVCM denotes a force generated by the voice coil motor. L2 denotes the distance between the rotation center of the actuator and the center of the voice coil motor.
FVCM is expressed by:FVCM=2BNIVCML  (3)where B denotes the intensity of magnetic fields generated by the magnet of the voice coil motor. N denotes the number of turns in a coil (voice coil) of the voice coil motor. L denotes the effective length of the coil. IVCM denotes a driving current (VCM current) flowing through the voice coil motor.
On the basis of equations (2) and (3), TVCM is expressed by:TVCM=FVCML2=2BNIVCMLL2  (4)
As is apparent from equation (4), TVCM is proportional to the current IVCM. A coefficient for the proportion of TVCM to the current IVCM (2BNLL2) is called a torque constant. Here, the intensity B of electric fields is generally proportional to the thickness of the magnet. In other words, in equation (4), B, L, and L2 are elements relating to the length (device scale). Accordingly, TVCM is proportional to the third power of the length (device scale).
On the other hand, if a suspension load is fixed, FU is not dependent on the length. Accordingly, TU expressed by equation (1) is proportional to the first power of the device scale. In this manner, TVCM is proportional to the third power of the device scale, whereas TU is proportional to the first power of the device scale. Thus, when an attempt is made to reduce the size of the hard disk drive (for example, from 2.5-inch type to 1-inch type), a decrease in TVCM is larger than that in TU. In other words, TU (the torque applied to the actuator during a head unload operation) is relatively larger than TVCM (the torque generated by the voice coil motor).
As described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-155455, the terminal voltage EVCM of the coil (voice coil) of the voice coil motor (that is, the terminal voltage of the voice coil motor) is expressed by:EVCM=RIVCM+ΔE  (5)where R denotes the resistance of the voice coil. Further, ΔE represents a back electromotive force voltage (back-EMF-voltage) generated by the voice coil.
The voice coil motor is driven by a motor driver (voice coil motor driver). A power supply voltage (driving voltage) for driving the motor driver is indicated by EP. The difference between EP and EVCM represents a voltage drop in the motor driver. Consequently, a power loss PLOSS in the motor driver is expressed by:PLOSS=(EP−EVCM)IVCM  (6)
As is apparent from equation (6), the power loss PLOSS in the motor driver decreases consistently with the difference between EP and EVCM. In general, in a seek operation of moving the head to a target track on the disk, the back-EMF-voltage ΔE is lower than a voltage drop RIVCM in the coil. Thus, the voice coil of the voice coil motor is designed so that the value of RIVCM during a seek operation nears the power supply voltage EP. This design enables the power loss in the motor driver to be minimized.
As described above, a reduction in the size of the hard disk drive increases the torque TU, applied to the actuator during a head unload operation, relatively to the torque TVCM, generated by the voice coil motor. To enable the head unload operation, it is necessary that TVCM is higher than TU. The torque that enables the head unload operation, that is, the torque to be generated by the voice coil motor so as to overcome the torque TU is called a “torque required for a head unload operation”. On the other hand, the torque that enables a seek operation is called a “torque required for a seek operation”. The torque required for the seek operation is not significantly affected by a reduction in the size of the hard disk drive. Thus, in a small-sized hard disk drive, the torque required for the head unload operation may be higher than that for the seek operation.
For such a hard disk drive, the voice coil of the voice coil motor must be designed in accordance with the current (VCM current) IVCM required for a seek operation in order to minimize the power loss in the motor driver. However, such a voice coil design may prevent the required VCM current from flowing through the voice coil during a head unload operation. In other words, the voice coil motor cannot generate a torque required for the head unload operation.
Thus, in the prior art, the voice coil of the voice coil motor is designed so as to ensure the torque required for a head unload operation. Specifically, the resistance R of the voice coil is reduced to allow more VCM current to flow through the voice coil during the head unload operation. However, a decrease in the resistance R of the voice coil requires the wire diameter of the coil to be increased to reduce the number of turns in the coil. In this case, the torque constant, that is, the coefficient of proportion (2BNLL2) decreases. In such a state, more VCM current must flow even during a seek operation. This may disadvantageously result in an increase in the power consumption of the motor driver during the seek operation.
Jpn. Pat. Appln. Publication No. 2001-155450 discloses a technique to switch a power supply voltage for the voice coil motor in order to reduce the power loss in the voice coil motor driver. This technique switches the power supply voltage for the voice coil motor between a seek operation and a tracking (following) operation. The seek operation requires much VCM current (torque), whereas the tracking (following) operation requires only a little VCM current (torque). On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 5-49291 discloses a technique to utilize a capacitor in which charges are accumulated as a power supply for a voice coil motor controller while the power supply is interrupted. A voltage boosted by a voltage booster is used to accumulate charges in the capacitor. This technique enables a head unload operation even while the power supply is interrupted.
As described above, the torque TU, applied to the actuator during a head unload operation, is proportional to the length of the arm of the actuator and thus varies in accordance with the first power of the device scale of the hard disk drive. On the other hand, the torque TVCM, which can be generated by the voice coil motor, varies in accordance with almost the third power of the device scale. Thus, if the size of the hard disk drive is reduced, the torque TU increases relatively to the torque TVCM. Here, the torque required for a head unload operation is assumed to be larger than that for a seek operation as in the case of a small-sized hard disk drive. In this case, the coil (voice coil motor) of the voice coil motor must be designed so that the voice coil motor can generate the torque required for the head unload operation. However, when the voice coil motor is designed in accordance with the torque required for the head unload operation, the power consumption of the motor driver increases during a seek operation. Further, the seek operation is performed many more times than the head unload operation.